The chafer component and outer sidewall rubber layer are important components of a pneumatic tire. The chafer component is important for connecting the tire to a rigid wheel rim and the outer rubber sidewall layer is important for protecting the tire sidewall which experiences atmospheric conditions during its normally extensive flexing as the tire is driven. Both tire components can, if desired, beneficially contribute to a reduction in internal heat generation and ultimate rolling resistance of the tire.
The chafer component is a rubber layer located in the bead region, or portion, of the tire which is used to seat the tire against a rigid wheel rim to form a tire/wheel assembly.
The outer sidewall rubber layer is located on the outer, atmospherically exposed, portion of the tire sidewall. The outer sidewall rubber layer (component) does not contain cord reinforcement. The tire's outer sidewall rubber layer refers to an outer tire sidewall rubber layer normally positioned on an outer portion of the tire sidewall, and therefore is a rubber layer which overlays and is not composed of the entire sidewall structure itself, and is exclusive of an apex or internal sidewall stiffening element contained in the tire sidewall, so that it is therefore outside of (axially outward from) the tire carcass ply layers, which themselves contain cord reinforcement. In this manner, the outer sidewall rubber layer is both usually visually observable and atmospherically exposed. Such sidewall outer rubber layer is usually subject to extensive flexing during the operation of the tire as is known to those having skill in such art and may be subject to, for example, one or more of fatigue resistance, cut growth resistance and ozone degradation resistance.
In practice, a pneumatic tire may be provided with a composite comprised of a chafer component and sidewall outer rubber layer as a co-extruded, unitary, composite with each of the chafer and adjoining rubber outer sidewall layer being comprised of the same or different rubber compositions. The composite is sulfur cured with the remainder of the rubber tire components to form an integral composite of the adjoining chafer and outer tire sidewall rubber layers.
A challenge is presented for providing a composite composed of both the chafer and outer sidewall rubber layer components for a tire intended for heavy duty service which are individually composed of rubber compositions which promote a reduction in hysteresis (e.g. reduction in hysteresis energy loss and thereby a reduction in internal heat generation during service, ultimately with a predictive promotion of beneficial relative lower rolling resistance for a pneumatic rubber tire with such composite).
A reduction in hysteresis is generally evidenced by an increase in its rebound physical property to thereby beneficially promote a reduction in internal heat generation within the chafer and outer sidewall components during the working of the tire during associated vehicular operation, particularly for heavy duty service of the tire.
In other words, lower hysteretic energy loss is typically evidenced by a higher rebound physical property of the rubber composition. Lower hysteretic energy loss is predictive of a beneficially lower rolling resistance of a tire with having such composite of chafer and outer sidewall rubber components with a predictive consequentially beneficial increased fuel economy for an associated vehicle.
To promote a higher rebound physical property of the rubber composition of the outer sidewall rubber layer or chafer component, (e.g. to promote a lower hysteresis property of the rubber composition), a reduction in its reinforcing filler content may be used such as, for example, a reduction in its rubber reinforcing carbon black content.
However, reducing the tire sidewall outer rubber layer's hysteresis by reducing its rubber reinforcing carbon black content may also result in a reduction of one or more other desirable physical properties such as, for example, a stiffness property.
It is therefore desirable to evaluate a means of providing a composite of individual outer tire sidewall rubber layer and chafer components with an increased rebound physical properties which is predictive of reduced hysteresis physical properties, as well as a desirable reduction in their tangent delta (tan delta) physical properties, promote an associated reduction in internally induced heat buildup with a desirable consequentially predictive beneficially reduced rolling resistance for a tire with an associated vehicular increase in fuel economy.
Accordingly, for such challenge, an evaluation is to be undertaken of providing a composite of a combination of integrally adjoining chafer and outer sidewall components for the tire with individual rubber compositions for each of the components containing the same or different specialized polybutadiene elastomers.
As indicated, a purpose of such undertaking is to promote a reduced hysteresis property (reduced hysteresis energy loss) loss for both of the sidewall and chafer rubber compositions.
In practice, elastomers for an outer tire sidewall rubber layer and chafer component are often comprised of a mixture, or blend, of natural cis 1,4-polyisoprene rubber and cis 1,4-polybutadiene rubber. Particulate reinforcement for the rubber composition in a form of rubber reinforcing carbon black, and sometimes precipitated silica, is often used
For this invention, use of specialized cis 1,4-polybutadiene elastomers is to be evaluated.
The specialized cis 1,4-polybutadiene elastomers are comprised of first and second cis 1,4-polybutadiene elastomers prepared by organic solution polymerization of 1,3-butadiene monomer in the presence of a neodymium based catalyst.
For the tire outer sidewall rubber layer component, use of a first specialized cis 1,4-polybutadiene rubber (elastomer) to be evaluated having a microstructure containing about 96 to about 99 percent cis 1,4-isomeric units, a vinyl content in a range of from about 0.2 to about 0.8 percent, a number average molecular weight (Mn) in a range of from about 200,000 to about 350,000, with less than 10, alternately about zero, percent having a number average molecular weight of less than 100,000, and with a Mw/Mn heterogeneity index (ratio of weight average molecular weight to number average molecular weight) in a range of from about 1.4/1 to about 2.1/1.
For the tire chafer component, use of a second specialized cis 1,4-polybutadiene rubber (elastomer) is to be evaluated having a microstructure containing about 96 to about 99 percent cis 1,4-isomeric units, a vinyl content in a range of from about 0.2 to about 0.8 percent, a number average molecular weight (Mn) in a range of from about 150,000 to about 300,000, with less than 10, alternately in a range of from about 2 to about 10, percent having a number average molecular weight of less than 100,000, and with a Mw/Mn heterogeneity index (ratio of weight average molecular weight to number average molecular weight) in a range of from about 1.4/1 to about 2.1/1 (a relatively moderate heterogeneity index range illustrating a moderate disparity between its number average and weight average molecular weights).
In a desirable embodiment the molecular weight (Mn) of the first specialized cis 1,4-polybutadiene rubber is greater than the molecular weight (Mn) of the second specialized cis 1,4-polybutadiene rubber such as, for example, wherein the molecular weight (Mn) of said first specialized cis 1,4-polybutadiene rubber is at least 30,000 greater than the molecular weight (Mn) of said second specialized cis 1,4-polybutadiene rubber.
In a desirable embodiment, the low average molecular weight (Mn) content (content below an Mn of 100,000) of the second specialized cis 1,4-polybutadiene rubber is greater than such low average molecular weight (Mn) content of the first specialized cis 1,4-polylbutadiene rubber by at least 2 percentage points such as, for example where the average low molecular weight (Mn) content below 100,000 for the first specialized cis 1,4-polybutadiene rubber is about zero percent, then such average low molecular weight Mn content below 100,000 for the second specialized cis 1,4-polybutadiene rubber is at least 2 percent and therefore at least 2 percentage points.
Historically, various polybutadiene elastomers (and polyisoprene elastomers) based on polymerizing 1,3-butadiene monomer with various neodymnium catalyst combinations have been previously presented. For example, and not intended to be limiting, see U.S. Pat. Nos. 3,794,604 and 6,780,948.
Historically, various tire components, including tire sidewalls, have been suggested which contain polybutadiene elastomers based on polymerizing 1,3-butadiene monomer with various neodymnium catalyst combinations.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.